The present invention relates to a converging position detecting apparatus, in particular, of a type which detects a deviation of a converging position on a scanning surface of a light beam that is emitted from a light source and scans the scanning surface in a converged state.
In conventional scanning optical apparatuses, a laser beam modulated in accordance with an image signal is deflected by a deflector to scan a scanning surface (e.g., the surface of a photoreceptor body) while being converged into a spot image by a converging lens system, to thereby effect recording. These apparatuses are commonly used in, for instance, laser printers. In these apparatuses, the laser beam converging state on the photoreceptor body is detected by a photoelectric conversion element disposed at an end of a scanning line.
In the above scanning optical apparatuses, environmental changes such as a temperature change may cause such problems as thermal deformation in individual parts of the lens system, Light source device, etc., to deviate the converging position of the laser beam perpendicularly to the scanning surface. As a result, the beam diameter (spot size) is increased to lower the image quality.
For example, Japanese Patent Application Unexamined Publication Nos. Hei. 3-65920 and Hei. 1-237614 disclose techniques to solve the above problem.
In an apparatus disclosed in the publication 3-65920, as shown in FIG. 5, a laser beam converging state on a scanning surface is detected by using laser beams reflected at monitoring points 27 and 28. More specifically, the laser beams reflected at the monitoring points 27 and 28 are collimated by respective collimating lenses 21 and 22, respectively reflected by a mirror 23 and a half mirror 24, and then converged onto a one-dimensional image pickup element 26 by an image forming lens 25. An inclination of an image surface 29 is detected by comparing laser beam converging states on the monitoring points 21 and 22.
In an apparatus disclosed in the publication 1-237614, laser beam converging position is detected by two spot size detecting means arranged approximately along the scanning direction spaced from each other perpendicularly to the scanning surface with the laser beam converging position interposed in between.
However, the above-described conventional apparatuses have the following problems.
In the scanning optical apparatus shown in FIG. 5, the laser beams reflected at the monitoring points 27 and 28 are monitored. Therefore, although beam diameters at the monitoring points 27 and 28 are determined, it cannot be determined whether the laser beam converging position is on the scanning surface or deviated from it perpendicularly. As a result, the deviation of the converging position cannot be corrected.
Further, the scanning optical apparatus of FIG. 5 is complex in configuration because the lenses 21, 22 and 25, mirrors 23 and 24, two-dimensional image pickup element 26, etc. are needed to detect beam diameters. Cumbersome operations of positioning the above parts are needed in assembling the scanning optical apparatus. Cumbersome adjustments may be required also when the scanning optical apparatus is in operation, when the respective parts are influenced by vibration, impact or environmental changes such as a temperature change. The deviation of the converging position is likely to happen in this apparatus.
In the second conventional apparatus, the converging position is detected by the two spot size detecting means, which are located at the one end portion on the scanning line. Therefore, the inclination of the image surface from the scanning surface cannot be known.